The major membrane phospholipids--lecithin, sphingomyelin, and phosphatidyl ethanolamine--have been shown to form spontaneously single bilayers at the water surface. Mixtures of these lipids will also form surface bilayers, but at temperatures which are intermediate between the temperatures at which the pure compounds form surface bilayers. The surface bilayers form only when in the presence of bulk lipid. The process of formation of the surface bilayers parallels the formation of membrane bilayers in that for both systems the saturated fatty acid composition increases when temperatures are elevated. The significance of this correlation as a possible mechanism for membrane biogenesis is being tested by studying the growth characteristics of the temperature sensitive bacillus stearothermophilus.